On Thu, Aug 02, 2018 at 10:24:57AM -0500, Benjamin Herrenschmidt wrote: > On Wed, 2018-08-01 at 01:36 -0700, Christoph Hellwig wrote: > > We just need to figure out how to deal with devices that deviate > > from the default. One things is that VIRTIO_F_IOMMU_PLATFORM really > > should become VIRTIO_F_PLATFORM_DMA to cover the cases of non-iommu > > dma tweaks (offsets, cache flushing), which seems well in spirit of > > the original design. > > I don't completely agree: > > 1 - VIRTIO_F_IOMMU_PLATFORM is a property of the "other side", ie qemu > for example. It indicates that the peer bypasses the normal platform > iommu. The platform code in the guest has no real way to know that this > is the case, this is a specific "feature" of the qemu implementation. > > 2 - VIRTIO_F_PLATFORM_DMA (or whatever you want to call it), is a > property of the guest platform itself (not qemu), there's no way the > "peer" can advertize it via the virtio negociated flags. At least for > us. I don't know for sure whether that would be workable for the ARM > case. In our case, qemu has no idea at VM creation time that the VM > will turn itself into a secure VM and thus will require bounce > buffering for IOs (including virtio). > > So unless we have another hook for the arch code to set > VIRTIO_F_PLATFORM_DMA on selected (or all) virtio devices from the > guest itself, I don't see that as a way to deal with it. > > > The other issue is VIRTIO_F_IO_BARRIER > > which is very vaguely defined, and which needs a better definition. > > And last but not least we'll need some text explaining the challenges > > of hardware devices - I think VIRTIO_F_PLATFORM_DMA + VIRTIO_F_IO_BARRIER > > is what would basically cover them, but a good description including > > an explanation of why these matter. > > Ben. > So is it true that from qemu point of view there is nothing special going on? You pass in a PA, host writes there. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
